Abstract
This study investigates the dosimetry and radiobiological model variation when a second photon arc was added to prostate volumetric‐modulated arc therapy (VMAT) using the single‐arc technique. Dosimetry and radiobiological model comparison between the single‐arc and double‐arc prostate VMAT plans were performed on five patients with prostate volumes ranging from 29−68.1 cm3. The prescription dose was 78 Gy/39 fractions and the photon beam energy was 6 MV. Dose‐volume histogram, mean and maximum dose of targets (planning and clinical target volume) and normal tissues (rectum, bladder and femoral heads), dose‐volume criteria in the treatment plan (D99% of PTV; D30%,D50%,V17Gy and V35Gy of rectum and bladder; D5% of femoral heads), and dose profiles along the vertical and horizontal axis crossing the isocenter were determined using the single‐arc and double‐arc VMAT technique. For comparison, the monitor unit based on the RapidArc delivery method, prostate tumor control probability (TCP), and rectal normal tissue complication probability (NTCP) based on the Lyman‐Burman‐Kutcher algorithm were calculated. It was found that though the double‐arc technique required almost double the treatment time than the single‐arc, the double‐arc plan provided a better rectal and bladder dose‐volume criteria by shifting the delivered dose in the patient from the anterior–posterior direction to the lateral. As the femoral head was less radiosensitive than the rectum and bladder, the double‐arc technique resulted in a prostate VMAT plan with better prostate coverage and rectal dose‐volume criteria compared to the single‐arc. The prostate TCP of the double‐arc plan was found slightly increased (0.16%) compared to the single‐arc. Therefore, when the rectal dose‐volume criteria are very difficult to achieve in a single‐arc prostate VMAT plan, it is worthwhile to consider the double‐arc technique.PACS number: 87.55.D‐, 87.55.dk, 87.55.K‐, 87.55.Qr
Highlights
Introductionequivalent or even better target coverage and normal tissue (rectum, bladder and femoral heads) sparing.[7,8,9,10,11] unlike step-and-shoot intensity-modulated radiotherapy (IMRT), prostate volumetric-modulated arc therapy (VMAT) interplays more dose delivery parameters such as dynamic multileaf movement, dose rate, and gantry speed within a single or multiple photon arcs in the treatment.[12,13,14,15] This complex delivery technique, requires more dedicated machine and patient quality assurance procedure, monitor unit (MU) calculation algorithm, and dosimetric evaluation (such as when patient size reduction due to weight loss) in the treatment.[16,17,18,19]
Average cumulated dose-volume histograms (DVHs) of the planning target volume (PTV), rectum, bladder, and left and right femoral head are shown in Figs. 1(a) to 1(e), planned using the single-arc and double-arc technique
It is found in volumetric-modulated arc therapy (VMAT) plans that the double-arc technique can lower the dose-volume criteria of the rectum and bladder (e.g., D30%, D50%, V17Gy, and V35Gy) but increase the rectal normal tissue complication probability (NTCP)
Summary
equivalent or even better target coverage and normal tissue (rectum, bladder and femoral heads) sparing.[7,8,9,10,11] unlike step-and-shoot IMRT, prostate VMAT interplays more dose delivery parameters such as dynamic multileaf movement, dose rate, and gantry speed within a single or multiple photon arcs in the treatment.[12,13,14,15] This complex delivery technique, requires more dedicated machine and patient quality assurance procedure, MU calculation algorithm, and dosimetric evaluation (such as when patient size reduction due to weight loss) in the treatment.[16,17,18,19]. From a retrospective planning study, Guckenberger et al[7] compared the dose-volume criteria among prostate step-and-shoot IMRT, single-arc, and multiple-arc VMAT They concluded that the multiple-arc prostate VMAT had a better dosimetric result than the single-arc at a cost of increased delivery time, MU, and spread of low doses. In the dosimetry comparison performed by Sze et al,(20) the authors reported that though the single-arc technique was more efficient regarding the delivery time and MU, it resulted in a higher rectal dose compared to the double-arc. They concluded that for a busy treatment unit, the single-arc technique could be an acceptable option provided that all planning dose-volume criteria were fulfilled. Results in this study should help medical physicists to understand the rationale of using more than one arc in the double-arc prostate VMAT plan
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